Douglas Carswell’s defection reminds us that it's the Tories who have the most to fear from Ukip

After a beaming Nigel Farage was chosen as Ukip candidate for South Thanet on Tuesday, Conservative ministers scoffed as he predicted that his party could hold the balance of power after next May’s general election. “Fantasy land,” one laughed.

They are not laughing now. Douglas Carswell’s dramatic defection from the Tories to Ukip has moved the tectonic plates at Westminster. There is a real prospect of Ukip securing its first elected MP in the October by-election Mr Carswell has triggered in his Clacton constituency. It is premature to say this is the start of a historic split in the Conservative Party, the realignment on the right that some Tories hope for. But that is now one scenario.

Ukip hopes that other Tories will defect if Mr Carswell retains his seat, as I suspect he will. Although several Tory MPs are talking to Ukip, it is far from certain that any more will jump ship soon. Mr Carswell has set the bar higher by bravely calling a by-election: other Tories who join Mr Farage’s “People’s Army” will be under enormous pressure to follow suit.

Some may not fancy their chances of winning a by-election or want to subject themselves to the pressure. Another prospect is a handful of Tory defections to Ukip in the run-up to the general election, when it would be too late to stage a by-election. A third scenario is that Ukip wins a handful of seats next May – no longer a remote possibility – and that some MPs elected as Tories then switch to Ukip, feeling some safety in numbers.

Mr Carswell’s unexpected bomb blew a hole in David Cameron’s strategy on Europe. The Prime Minister was banking on his troublesome Eurosceptics keeping their powder dry until after the general election, because of his promise of an in/out referendum on Europe in 2017. Mr Cameron’s advisers were hopeful that this pledge, made in January 2013, would keep Europe out of the spotlight and shoot Ukip’s fox. It did nothing of the sort. Mr Farage’s party topped the poll in the European Parliament elections in May. Since then, Ukip’s rating in The Independent’s poll of polls dropped from 15 to 13 per cent as it won fewer headlines, but the recruitment of Mr Carswell will ensure a fresh supply of the oxygen of publicity Ukip desperately needs.

Mr Cameron’s biggest electoral task – wooing back the natural Tory supporters who have switched to Ukip – got a lot harder this week. He hoped his trump card would be a message saying: “Vote Ukip, get Miliband” – in other words, Ukip could take enough votes from the Tories in key marginals to allow Mr Miliband into Downing Street through the back door. This part of the Cameron strategy was always open to question: surveys by Lord Ashcroft, the former Tory deputy chairman, found that only one in 10 ex-Tory supporters now backing Ukip would be swayed by such warnings.

Mr Carswell has made it even harder for this approach to work. His discussions with Cameron aides persuaded him that the PM is determined to recommend that Britain stay in the EU in the referendum, even if he does not win big changes to membership terms in his renegotiation. If Mr Cameron is not believed by Mr Carswell, who backed him in the 2005 Tory leadership contest, then how on earth will he persuade the voters to trust him?

Tory Eurosceptics, including some Cabinet ministers, will now press Mr Cameron to declare that he would be prepared to urge an “out” vote if he fails to win a good enough new EU deal. He may announce this at the Tory conference next month. The downside is that the move may reduce his chances of winning concessions from his EU counterparts. “Threats to leave the club are not how to get what you want,” one EU diplomat told me. “It is not the European way.”

The Eurosceptics will also renew their demands on Mr Cameron to set out his shopping list of demands. This path would also be strewn with thorns. Only one thing is certain about the ravenous Europhobes: their appetite is never satisfied (the referendum pledge being a classic example). Some Eurosceptics would inevitably view whatever Mr Cameron demanded as not big enough. Meanwhile, other EU members would respond to his proposals, and some would undoubtedly say “No”, fuelling Mr Carswell’s claims that the great renegotiation will prove a “damp squib”.

Ukip’s fresh momentum is not only bad news for the Tories. The Lib Dems are resigned to losing yet another deposit in the Clacton by-election, their role as the protest party seized by the Farage army. Labour enjoyed the Carswell-Farage show but has little to crow about. Labour held Mr Carswell’s former seat of Harwich between 1997 and 2005, but will not put much effort into the by-election, which it views as a two-horse race between Ukip and the Tories.

This is quite a statement about an official Opposition, a revealing insight into how Ukip has changed the game. Its momentum could now run until next May, posing a threat to Labour in some of its northern heartlands. Watch out for a Ukip surge and backlash against Labour in Rotherham after this week’s horrific revelations about the abuse of girls. Ukip’s annual conference next month will be held in Mr Miliband’s backyard in Doncaster.

Yet Mr Carswell’s defection reminds us that it is the Tories who have the most to fear from the Purple Peril. If the Ukip tide continues to rise, it will make a Labour-led government next May more likely.